Mold material for casting metal

ABSTRACT

Disclosed is a mold material for metal casting capable of suppressing the deterioration of cast metal. The mold material for metal casting contains hemihydrate gypsum and a heat resistant material, wherein an oxoacid salt of phosphorus, insoluble in water, is contained in the mold material.

FIELD OF THE INVENTION

[0001] This invention relates to a gypsum-based mold material for casting metal and, more particularly, to a mold material for casting metal in which it is possible to prevent deterioration of cast metal. The invention also relates to a method for casting a metal casting using a gypsum-based mold.

RELATED ART

[0002] In the circles of ornamentation and in the dental circles, a so-called lost wax method casting is used for casting metal in order to reproduce complicated shapes. In the lost wax method, a pattern of a desired shape is formed, using wax. This wax pattern is set upright within a casting ring (mold frame). The mold material for metal casting and water are kneaded together to form a slurry, which slurry is caused to flow into a casting ring to cause a wax pattern to be buried therein. The slurry is then set and fired along with the ring at 700 to 800° C. This burns the wax off to form a cavity in the (investment) mold. Molten metal is then poured into the mold by a centrifugal method, a depressuring method or a pressurizing method etc. The resulting assembly is then allowed to cool and the mold is then removed to yield a cast product. The use of a gypsum mold gives such merits as improved fluidity in pouring the slurry into the casting frame, only small deformation otherwise caused by residual stress following the firing, improved takeout performance of the cast product, and only small chronological deterioration.

[0003] As a mold material for metal casting, a hemihydrate gypsum, as a binder, and an inorganic oxide, as a heat resistant material, such as alumina, spinel, crystobalite, quartz, mullite, zirconia and/or magnesia, are used as main components. In addition to these main components, auxiliary components, such as setting time adjustment agent, expansion agent, shrinkage-preventing agent, gas-permeability improver and/or water-reducing agent, may be used as additives, if necessary. For example, there is known a dental investment material (mold material for a precision metal casting) containing a binder (binding agent) and a refractory material (heat resistant material), added by aluminum phosphite (expansion agent) (see Patent Publication 1).

[0004] The metal used for casting is noble metal, in a majority of cases. For this reason, an excess amount of metal in a cast product is used as a mixture with fresh unused (remaining) metal for next casting.

[0005] Patent Publication 1: JP Patent Kokai JP-A-9-227326

[0006] Also there are a number of known methods for dental molding and casting as follows:

[0007] JP07-2617A,

[0008] JP2002-87918A,

[0009] JP59-181203A,

[0010] U.S. Pat No. 5,373,891,

[0011] U.S. Pat. No. 5,718,749,

[0012] U.S. Pat. No. 4,814,011,

[0013] U.S. Pat. No. 4,909,847, and

[0014] U.S. Pat. No. 5,304,239.

[0015] None of the prior art documents mentions the possibility of deterioration of the metal casting.

SUMMARY OF THE DISCLOSURE

[0016] However, with the gypsum-based mold material for metal casting, there are occasions where gases are generated during firing or casting and taken into molten metal, with the result that the metal is deteriorated and fails to exhibit the desirable properties.

[0017] That is, with the gypsum-based mold material for metal casting, gypsum (CaSO₄) is sometimes decomposed on heating. Theoretically, the gypsum of high purity is retained to be not decomposed up to 1000° C. However, the decomposition temperature is sometimes lowered to approximately 700° C., due to e.g. the addition of the auxiliary agents, such that, in the mold sites where gas-permeability is poor, there may be accumulated gases yielded on thermal decomposition of gypsum. Also, there are occasions where the gypsum decomposition occurs even in case the metal having a melting point of 1000° C. or higher is cast into the mold.

[0018] In the decomposition of gypsum, SO₃ or SO₂ gas, referred to below as SO_(x) gas, is yielded by a reaction of CaSO₄→CaO+SO_(x). If this gas is taken into molten metal, the metal is deteriorated and becomes brittle. Moreover, since any excess metal used in casting is mixed and melted with fresh unused metal, at the time of the next casting, the effect of the SO_(x) gas is cumulated with repeated casting, thus gradually embrittling the metal.

[0019] It is an object of the present invention to provide a mold material for metal casting whereby it is possible to suppress deterioration of metal cast.

[0020] The present invention provides a mold material for metal casting containing hemihydrate gypsum and heat resistant material, wherein an oxoacid salt of phosphorus, insoluble in water, is contained in the mold material. This assures suppression of deterioration of cast metal.

[0021] In the mold material for metal casting, according to the present invention, the oxoacid salt of phosphorus is preferably solid at 1000° C. or higher, preferably 1200° C. or above, most preferably 1300° C. or above.

[0022] In the mold material for metal casting, according to the present invention, the oxoacid salt of phosphorus is preferably at least one selected from orthophosphates, metaphosphates, diphosphates and phosphates, preferably at least one salt of Ca, Mg and/or Al, and most preferably, calcium diphosphate.

[0023] In the mold material for metal casting, according to the present invention, the heat resistant material is preferably selected from alumina, silica, spinel, zirconia, zircon, mullite, magnesia and calcia, and more preferably, alumina.

[0024] In the mold material for metal casting, according to the present invention, preferably the content of the hemihydrate gypsum is not less than 20 wt % and not higher than 80 wt % and the content of the oxoacid salt of phosphorus is not less than 5 wt % and not higher than 30 wt %.

BRIEF DESCRPTION OF THE DRAWINGS

[0025]FIG. 1 is a first graph showing the relationship between the number of times of casting and the angles of bend of the cast metal samples obtained by casting from respective mold materials for metal casting containing different types of the oxoacid salt of phosphorus in respective different amounts.

[0026]FIG. 2 is a second graph showing the relationship between the number of times of casting and the angles of bend of the cast metal samples obtained by casting from respective mold materials for metal casting containing different types of the oxoacid salt of phosphorus in respective different amounts.

PREFERRED EMBODIMENTS OF THE INVENTION

[0027] The mold material for metal casting, embodying the present invention, is now explained. The mold material for metal casting, embodying the present invention, contains at least hemihydrate gypsum, a heat resistant material, and an oxoacid salt of phosphorus.

[0028] The hemihydrate gypsum is a binder obtained as a hemihydrate by firing gypsum dihydrate, as a starting material, under atmospheric pressure or under pressure application. When hydrated, the hemihydrate gypsum is set by forming an entwined structure of needle-like crystals of gypsum dihydrate, and thus acts as a binder for the mold. There are two sorts of the hemihydrate gypsum, namely an α type and a β type. The content of the hemihydrate gypsum is 20 to 80 wt %. With a content of the hemihydrate gypsum less than 20 wt %, the mold is low in strength, such that cracks tend to be formed due to shortage in strength following the sintering. In addition, the mold tends to expand excessively on heating. On the other hand, with a content of the hemihydrate gypsum exceeding 80 wt %, there is a risk that the expansion is not up to such a level that compensates for the casting shrinkage of metal to be cast. The content of the hemihydrate gypsum is preferably 20 to 60 wt % and more preferably 20 to 40 wt %.

[0029] The heat resistant material imparts properties of withstanding elevated temperatures, such as during casting of molten metal. The heat resistant material may be enumerated by alumina, silica (quartz, cristobalite, tridymite), spinel, zirconia, zircon, mullite, magnesia and calcia etc. Of these, alumina is preferred.

[0030] The oxoacid salt of phosphorus is an additive suppressing the deterioration of metal cast into the casting mold. The oxoacid salt of phosphorus, insoluble (hardly soluble) in water, is selected. The oxoacid salt of phosphorus, solid at a temperature not lower than 1000° C., is selected. The oxoacid salt of phosphorus used is preferably solid at a temperature not lower than 1200° C., and more preferably solid at a temperature not lower than 1300° C. It is noted that the oxoacid salt of phosphorus is not used as a binder. The oxoacid salt of phosphorus may be enumerated by orthophosphates, metaphosphates, diphosphates and phosphates etc., and may, for example, be aluminum metaphosphate, magnesium diphosphate, calcium diphosphate and/or calcium phosphate (or 4CaO—P₂O₅). As calcium phosphate, pulverized bone china, regenerated calcium phosphate (bi-product of the glue producing process) and bone ash etc. may be used. The content of the oxoacid salt of phosphorus is 5 to 30 wt %. With a content of the oxoacid salt of phosphorus less than 5 wt %, it may become impossible to suppress deterioration of the metal to be cast. On the other hand, with the content of the oxoacid salt of phosphorus in excess of 30 wt %, the setting properties, such as setting time or setting expansion, tend to be affected adversely. The content of the oxoacid salt of phosphorus is preferably 5 to 25 wt % and more preferably 10 to 25 wt %. The average particle size of the oxoacid salt of phosphorus is not larger than 100 μm, preferably not larger than 701m and more preferably not larger than 50 μm.

[0031] The mold material for metal casting of the present embodiment is preferably added by a shrinkage-suppressing agent, an expansion agent, a water reducing agent (i.e., agent for reducing the amount of water) and a timing adjustment agent, in order to produce a casting mold of high precision and a desired shape.

[0032] The casting mold for metal casting of the present embodiment is produced as follows:

[0033] A wax pattern, formed by a pattern part having a desired pattern, such as a tooth (or crown) pattern, and a support part carrying the pattern part, is set upright in a mold frame. A mold material (usually called “investment material”) for metal casting is poured into the mold frame so that the wax pattern portion except for an end portion of the support part is embedded (immersed) in the mold material (slurry) for metal casting. The mold material for metal casting is then set. Finally, the so hardened mold material for metal casting is sintered by firing, and the wax pattern is gasified to produce a mold having a cavity of the desired pattern.

[0034] The pattern part of the wax pattern is shaped, e.g., by taking (impressing) a tooth pattern (configuration) of a patient, using an impression (plastic) material, taking the tooth pattern by preparing a model made of a gypsum for model, repeatedly melting and solidifying wax on the gypsum mold, followed by trimming and modifying the shape. The support part provided to the wax pattern is configured so as to support the pattern part in the mold frame and, after pouring a slurry of a mold material into the mold frame, the support part is gasified on firing to form a cavity of the same shape as the desired pattern (e.g. of tooth crown). Although the wax pattern is embedded in the mold (investment) material, an end of the support part of the wax pattern is exposed out of the mold material, in order to allow the wax pattern to flow out through the so exposed portion during firing of the mold material.

[0035] The mold material is added with a suitable amount of water and cast into the mold frame. The mold material is then allowed to stand and thereby set.

[0036] The firing temperature for the mold material needs to be such a temperature that allows the wax pattern to be gasified. Specifically, the desirable firing temperature is 700 to 850° C. With a firing temperature lower than 700° C., the wax tends to be left. On the other hand, with a firing temperature exceeding 850° C., the gypsum contained in the mold material tends to be decomposed.

EXAMPLES

[0037] Several Examples of the present invention are hereinafter explained. A mold was prepared, by a lost wax method, from a starting material composed of 100 parts by weight of a mold material for metal casting, having compositions shown in Table 1, not more than 0.1 part by weight of a setting time adjustment agent and 0.7 part by weight of the water reducing agent. Using the so produced mold, metal was cast and measurements were made for the properties of the resulting cast metal. Turning to the average particle size of the oxoacid salt of phosphorus, that of Al(PO₃)₃ is 6.7 μm, that of Mg₂P₂O₇ is 27 μm, and that of Ca₂P₂O₇ is 64 μm. TABLE 1 amount of added oxoacid sort of oxoacid salt of hemihydrate of salt of phosphorus gypsum alumina samples phosphorus (wt %) (wt %) (wt %) Ex. 1 Al(PO₃)₃ 10 30 60 Ex. 2 Mg₂P₂O₇ 10 30 60 Ex. 3 Ca₂P₂O₇ 20 30 50 Ex. 4 Ca₂P₂O₇ 10 30 60 Ex. 5 Ca₂P₂O₇ 5 30 65 Comp. Ex. 1 Ca₂P₂O₇ 2 30 68 Comp. Ex. 2 none 0 30 70 Ex. 6 Ca₂P₂O₇ 10 20 70 Ex. 7 Ca₂P₂O₇ 10 30 60 Ex. 8 Ca₂P₂O₇ 10 40 50

[0038] The conditions for producing the mold are now explained. As for the mold material for metal casting, each 100 g of a mold material for metal casting (containing a setting time adjustment agent and a water reducing agent) as shown in Table 1, was added with 28 g of water, and the resulting mixture was disintegrated in a stirrer vessel until a homogeneous mass was obtained. This homogeneous mass was stirred for one minute by a vacuum stirrer, using a vacuum stirrer for dental use, to form a slurry. As for the wax pattern, a sheet of plastics, 0.5×7×30 mm in size, was used in place of the wax pattern. The setting time of the mold material for metal casting was on the order of 30 minutes. The firing conditions for the mold material for metal casting were 800° C. for 60 minutes. The hemihydrate gypsum used was a powder obtained by pulverizing the α-type hemihydrate gypsum AT-20 manufactured by Toray Co., Ltd. The purity was 98% or above.

[0039] As for metal, a noble metal alloy for baking the porcelain for dental, manufactured by NORITAKE Co. Ltd. (trade name: P-60, Pd:60%, Ag:27%, In:8% and other ingredient:5%) was used and melted at approximately 1350° C. by an electrical resistance heat source. The melted metal was cast using a pressure reducing/pressurizing casting machine.

[0040] The method for measuring characteristics of a cast product is now explained. A cast product, obtained by the lost wax method, was secured at one end and bent at the other end with pliers. The angle of bend when the bent part was fractured was measured by visual check. After the measurement, the cast product was melted and a cast product was produced by the same method to make similar measurements. This sequence of steps was repeated to make measurements four times. That is, four measurements were carried out per each single sort of the mold material for metal casting. The measured results are shown in the following Table. Meanwhile, unused fresh metal was used for the first casting.

[0041] The method for evaluating characteristics of the cast product is now explained. It is defined that the smaller the angle of bend measured by the above method, the more brittle is to be the cast product. That is, the fact that the smaller this angle of bend, the more severely deteriorated can be judged that the deterioration of metal has proceeded under the influence of the SO_(x) gas yielded due to decomposition of gypsum. The angles of bend per each casting of the cast materials obtained by the molds of respective samples of Table 1 (mold material for metal casting) are shown in the following Table 2. TABLE 2 angle of bend (°) samples first second third fourth Ex. 1 80 78 70 80 Ex. 2 70 73 75 75 Ex. 3 75 73 75 75 Ex. 4 70 73 68 78 Ex. 5 70 75 65 70 Comp. Ex. 1 72 75 60 60 Comp. Ex. 2 73 80 55 58 Ex. 6 75 75 72 75 Ex. 7 70 73 68 78 Ex. 8 70 70 68 64

[0042] Turning to the Comparative Examples 1 and 2 of Table 2 and FIG. 1, the angle of bend fell to 60° or below for the third and subsequent casting operations such that the cast material becomes brittle, except the cases where the oxoacid salt of phosphorus is added to the mold material for metal casting. Meanwhile, with the Comparative Example 1, the angle of bend is still 60° for the third and the fourth casting, meaning that, in certain view, this Comparative Example might be qualified as an Example of the present invention. As contrasted thereto, with the Examples 1 to 8, in which the oxoacid salt of phosphorus is added to the samples, the angle of bend unexceptionally exceeds 60°, even for the fourth casting, as may be seen from Table 2 and from FIGS. 1 and 2, so that it is possible to prevent the angle of bend from being lowered. Thus, with the Examples 1 to 8, the addition of the oxoacid salt of phosphorus suppresses the yielding of the SO_(x) gas caused by decomposition of gypsum from the mold material for metal casting, thereby suppressing metal deterioration. In light of the above results, it can be understood that there would be a critical point in suppressing the deterioration of cast metals around the content of 2 to 5 wt % of the oxoacid salt of phosphorus in the mold material for metal casting.

[0043] The meritorious effects of the present invention are summariezed as follows.

[0044] According to the present invention, it is possible to suppress the generation of SO_(x) gas caused by decomposition of gypsum, at the time of the heating (firing or casting) of the mold material for metal casting, and hence to suppress the deterioration of metal.

[0045] It should be noted that other objects, features and aspects of the present invention will become apparent in the entire disclosure and that modifications may be done without departing the gist and scope of the present invention as disclosed herein and claimed as appended herewith.

[0046] Also it should be noted that any combination of the disclosed and/or claimed elements, matters and/or items may fall under the modifications aforementioned. 

What is claimed is:
 1. A mold material for metal casting, wherein said mold material comprises hemihydrate gypsum and heat resistant material, said mold material further comprising an oxoacid salt of phosphorus, which is insoluble in water.
 2. The mold material for metal casting as defined in claim 1, wherein said oxoacid salt of phosphorus is solid at 1000° C. or above.
 3. The mold material for metal casting as defined in claim 1, wherein said oxoacid salt of phosphorus is solid at 1200° C. or above.
 4. The mold material for metal casting as defined in claim 1, wherein said oxoacid salt of phosphorus is solid at 1300° C. or above.
 5. The mold material for metal casting as defined in claim 1, wherein said oxoacid salt of phosphorus comprises at least one selected from the group consisting of orthophosphates, metaphosphates, diphosphates and phosphates.
 6. The mold material for metal casting as defined in claim 1, wherein said oxoacid salt of phosphorus comprises at least one salt of a metal selected from the group consisting of Ca, Mg and/or Al.
 7. The mold material for metal casting as defined in claim 1, wherein said oxoacid salt of phosphorus comprises calcium diphosphate.
 8. The mold material for metal casting as defined in claim 1, wherein said heat resistant material comprises at least one selected from the group consisting of alumina, silica, spinel, zirconia, zircon, mullite, magnesia and calcia.
 9. The mold material for metal casting as defined in claim 1, wherein said heat resistant material is alumina.
 10. The mold material for metal casting as defined in claim 1, wherein said hemihydrate gypsum is present in an amount of 20 to 80 wt % and said oxoacid salt of phosphorus is present in an amount of 5 to 30 wt %.
 11. The mold material for metal casting as defined in claim 1, wherein said oxoacid salt of phosphorus is present in an amount of 5 to 25 wt %.
 12. The mold material for metal casting as defined in claim 1, wherein said oxoacid salt of phosphorus is present in an amount of 10 to 25 wt %.
 13. A method for casting a metal casting comprising the steps of: providing a mold material comprising hemihydrate gypsum, heat resistant material, and an oxoacid salt of phosphorus which is insoluble in water, preparing a hardened mold using said mold material through heating, and casting a molten metal into said mold to produce a casting of a metal.
 14. The method as defined in claim 13, wherein said mold material comprises 20 to 80 wt % of hemihydrate gypsum and 5 to 30 wt % of oxoacid salt of phosphorus.
 15. The method as defined claim 13, wherein said metal comprises noble metal. 